I cracked a red brick open and found it to be gray on the inside. Is this a problem?
Just because the brick is gray on the inside doesn't mean you have bad properties. Color alone doesn't tell whether it's good or bad. A gray color on the inside of brick may suggest incomplete firing or not enough oxygen at a certain firing stage. To verify that a brick was properly fired, have it tested in a lab or ask the brick manufacturer for test results. If the properties of the brick meet minimum requirements, the brick is acceptable.